Freeway air pollution travels farther in early morning

Traffic crawls through downtown Los Angeles on the 110 Freeway. New research shows that in the early morning, air pollution plumes from freeways can travel more than a mile downwind.

Traffic crawls through downtown Los Angeles on the 110 Freeway. New research shows that in the early morning, air pollution plumes from freeways can travel more than a mile downwind. (Robert Gauthier / Los Angeles Times)

Two years ago researchers outfitted an electric Toyota RAV4 with a set of test instruments and drove back and forth near four Los Angeles County freeways between 4:30 a.m. and 6:30 a.m., sampling the air.

The results confirmed that in the early morning, concentrated plumes of air pollution from freeways can travel more than a mile downwind, exposing more residents than previously thought to harmful pollution levels.

Most previous air quality studies, based on measurements taken during the day or evening, have found that vehicle emission plumes generally blow no more than about 1,000 feet downwind from a major roadway before they break up.

But in the hours just before sunrise, weather conditions are different. Nocturnal surface inversions, caused by nighttime cooling, trap air near the ground, slowing the dispersal of concentrated pollution particles and allowing them to travel farther than during the day.

A 2009 study documented extended emission plumes near the 10 Freeway in the early morning. To see whether the same thing was happening elsewhere, researchers from UCLA and the California Air Resources Board in 2011 sampled the air in residential neighborhoods downwind of the 91 Freeway in Paramount, the 210 in Claremont, the 110 in Carson and the 101 in downtown Los Angeles.

Their findings, published in December in the journal Atmospheric Environment, suggest that in the hours before sunrise, residential exposure to freeway pollution is more far-reaching than previously thought.

“It’s clear heavily trafficked roadways have a large impact on downwind populations, and a similar situation likely happens around the world in the early morning hours,” said Suzanne Paulson, a UCLA atmospheric sciences professor and co-author of the paper. “The particles tend to end up indoors, so a lot of people are being exposed inside their homes and schools.”

Studies have shown that exposure to elevated levels of vehicle pollution can contribute to asthma, heart disease and other health problems.

In greater Los Angeles, where apartment buildings and single-family homes stand cheek by jowl with some of the busiest freeways in the nation, the researchers estimated that on any given morning, roughly a quarter of the population could be exposed to downwind pollution consisting of ultrafine particles, nitric oxide and hydrocarbons.

Their advice: If you live within roughly a mile of a freeway and are downwind, keep your windows closed in the hours just before sunrise. Use air conditioning. Install HEPA air filters. Postpone outdoor exercise until later in the morning or exercise farther away from the highway.

After sunrise, the surface air warms up and the inversion breaks up, diluting the pollution.